Officials said they conducted two security sweeps during the marathon: one before the race, and one an hour before first finishers completed the race. They said there were no specific threats made before the marathon.

Late Monday night, state police said a search warrant related to the investigation was served at an apartment complex on Ocean Avenue in Revere, Mass.

According to ABC News, officials are tracking multiple persons of interest, including a man being treated at an area hospital and another individual who was seen trying to gain access into a restricted area five minutes before the blast.

Police said they have no suspects in custody. Security officials said there were no threats made before the marathon.

DesLauriers said the FBI will be working with federal, state and local investigators on the case.

Three people were killed in the deadly attack and at least 176 were injured, with 17 in critical condition.

Among the deceased was 8-year-old Martin Richard. His father, Bill Richard, released a statement Tuesday that his wife and daughter are recovering from serious injuries.

"We thank our family and friends, those we know and those we have never met, for their thoughts and prayers," Richard said.

WCVB-TV confirmed the identity of a second victim killed in the blasts as 29-year-old Krystle Campbell, a 2001 graduate of Medford High School in Massachusetts.

The name of the third victim hasn't been released, but Boston University said Tuesday evening that the victim was a graduate student there. The university said it does not yet have permission from the student's family to release the name.

Boston police said the explosions were caused by bomb blasts. The first bomb exploded about two hours after the winners crossed the finish line. The second explosion happened about 10 seconds later, a couple of blocks up the route.